I don't understand why bicycles have big front chainrings and smaller cogs in the rear.(typically)
It seems to me that if you are in a gear in which you're in a 39 front chainring and a 20 rear cog, then it should be essentially the same gear as being in a 42 front chainring and a 23 rear cog, is that not correct? If you look at an old singlespeed bike, or a fixie, the front ring is always significantly larger than the rear cog. When you consider chain wrap and how chains and sprockets/gears wear out when they are put under stresses associated with not having many teeth engaged when under power, doesn't it seem like it would make more sense to have the same sized sprockets on front and back to more evenly space out the load?
Even with cassettes and derailleur bikes it seems like the front chainrings are generally larger than most or all of the cassette cogs.
Motorcycles are opposite and typically have a smaller front drive sprocket and a larger rear wheel pulley, so I can't figure that out either.
I'm sure there's a simple explaination for this, can someone point me in the right direction?